Update README for better formatting of Mac app build instructions

Instructions were confusing because the newlines in the README.md file aren't rendered on Github.

## Before:
![readme-before](https://f.cloud.github.com/assets/178680/1335589/1f2a1486-35bb-11e3-8644-22b983877d63.png)

## After
![readme-after](https://f.cloud.github.com/assets/178680/1335592/28fdb6c0-35bb-11e3-934e-b513be7c6c32.png)